In Prorender there is a feature still missing compared to the Cycles renderer. In Cycles you can chose the option
RenderOptions->Film->Transparence and then tick the box "Transparent Glass", which makes glass materials let background images show through, when put on top.
In Prorender you have Film->Transparent as an option, which makes the scene rander with transparant background, but there is no way to let glass be transparent. Will that option be available also for Prorender any soon?